This invention is concerned with devices for transporting semiconductors during their fabrication.
In the manufacture of semiconductors, wafers of semi conductor material are placed in a tubular furnace for various treatment such as oxidation, diffusion, annealing and low pressure chemical vapor deposition. Typically, gaseous materials are generated for deposition on the wafers to form layers of different current type. The wafers are usually supported in parallel, one behind the other, and on their edges in a rack which is often referred to as a boat. The rack may be detachable from, or integral with, a support which serves to carry the rack, or a series of racks, into and out of the furnace. The term "carrier" will be used herein to mean either the support for one or more detachable racks or the support and rack when the two are inseparable. Since very high temperatures, about 1,000.degree. to 1,300.degree. C., are generated in the furnace, the carrier and furnace are made from the refractory materials such as silica or quarts, usually the latter.
It is of paramount importance in the manufacture of semiconductors that the materials, equipment and environment used be ultra pure. Any undesirable impurities can spoil the semiconductor. Clearly spoilage reduces the efficiency of the process, increases costs and waste and, if passed undetected, can lead to very expensive breakdown of equipment where the semiconductor is subsequently used.
Some wafer carriers are in the form of a sled for sliding in and out of the furnace. This design suffers from the disadvantage that the sliding can dislodge particles deposited on the walls of the furnace to contaminate the wafers. Other designs have wheels to roll the carrier in and out of the furnace, but wheels also present problems. Since such high temperatures are experienced in the furnace, normal wheel bearings and lubricants cannot be used. Without such aids to smooth the motion the moving parts of the carrier are in direct contact and since the carrier is made of materials such as quartz the wheels can stick, since the coefficient of friction between quartz is high. If the wheels stick, the carrier can slide, dislodging particulate contamination as mentioned above. Alternatively, or in addition, when the wheels stick, the carrier can move with a jerky motion which jostles the wafers to damage them or realign them so that all the wafers in a batch do not receive uniform treatment.
One approach in the prior art to overcome these problems has been to encapsulate the contact surfaces of the moving parts with graphite thus providing a sealed in lubricant, but this is expensive and complicated. Without lubricant or some form of bearing, there cannot be tight tolerances between the wheels and axles without binding. Alternatively a large tolerance may prevent the wheels from operating properly as they will flop to one side on the axles. Attempts have been made to avoid this by supporting the wheels with quartz washers on either side but this only increases the quartz contact area and hence the resistance to motion, and also provides traps where deposits can collect for contamination of the wafers. Another alternative is to use broad wheels of substantially the same width as the axles, but this also increases quartz to quartz contact and provides a large mass which increases the length of time for which the carrier and wafers must be cooled, thereby increasing processing time and costs.
There is therefore a need for a wheeled carrier of a relatively simple and inexpensive construction and which moves smoothly without sliding or jerking which does not trap contaminating particles.